In recent years, in order to improve the transportation efficiency of natural gas and oil, high-strength large-diameter heavy wall steel pipes that can withstand the high-pressure operation have been used for line pipes because of an increase in demand for energy. To meet the demand, hitherto, UOE steel pipes made of plates have been mainly used. Recently, however, a strong demand for a further reduction in the cost of pipeline construction, the undersupply of UOE steel pipes, and so forth have strongly required a reduction in the steel material cost of steel pipes. Thus, electric resistance welded steel pipes or tubes and spiral steel pipes, which are produced in higher productivity and less expensive than those of UOE steel pipes and which are made of hot-rolled steel sheets, have been used.
Here, pipelines are often constructed in cold weather regions with, for example, abundant natural gas reserves. Thus, steel sheets used as steel materials for line pipes are required to have high strength and excellent low-temperature toughness. Hitherto, electric resistance welded steel pipes or tubes and spiral steel pipes have been widely used for automotive members, steel pipe piles, and so forth and are typically made of hot-rolled steel sheets with a relatively small thickness. However, in the case where heavy wall steel pipes are required, it is necessary to use hot-rolled steel sheets with a larger thickness than ever before. In the case where steel sheets with a large thickness are produced, in particular, surface regions of steel sheets in the thickness direction are processed under severe conditions. Furthermore, line pipes constructed over long distances may be forcefully deformed by crustal change, such as an earthquake. Thus, hot-rolled steel sheets used as materials for line pipes are required to have elongation characteristics that can withstand the foregoing processing and deformation in terms of the overall thickness, in addition to desired strength and low-temperature toughness.
In light of the foregoing circumstances, nowadays, various techniques regarding hot-rolled steel materials for line pipes are reported.
For example, Patent Literature 1 reports a technique for providing a hot-rolled steel strip for a high-strength electric resistance welded steel pipe, the hot-rolled steel strip having excellent low-temperature toughness and weldability and having a composition which contains, on a mass % basis, 0.005% to 0.04% C, 0.05% to 0.3% Si, 0.5% to 2.0% Mn, 0.001% to 0.1% Al, 0.001% to 0.1% Nb, 0.001% to 0.1% V, 0.001% to 0.1% Ti, 0.03% or less P, 0.005% or less S, 0.006% or less N, one or more of 0.5% or less Cu, 0.5% or less Ni, and 0.5% or less Mo, and the balance being Fe and incidental impurities and in which when Pcm=[% C]+[% Si]/30+([% Mn]+[% Cu])/20+[% Ni]/60+[% Mo]/7+[% V]/10, Pcm is 0.17 or less, the hot-rolled steel strip having a microstructure that contains bainitic ferrite serving as a main phase, the bainitic ferrite accounting for 95% by volume or more in the whole microstructure.
Patent Literature 2 reports a technique for providing a heavy high-strength hot-rolled steel sheet having excellent low-temperature toughness and uniformity of a steel material in the thickness direction and having a composition which contains, on a mass % basis, 0.02% to 0.08% C, 0.01% to 0.50% Si, 0.5% to 1.8% Mn, 0.025% or less P, 0.005% or less S, 0.005% to 0.10% Al, 0.01% to 0.10% Nb, 0.001% to 0.05% Ti, and the balance being Fe and incidental impurities, C, Ti, and Nb being contained in such a manner that ([% Ti]+([% Nb]/2))/[% C]<4, the hot-rolled steel sheet having a microstructure in which the difference ΔD between the average grain size of a ferrite phase serving as a main phase at a position 1 mm from a surface of the steel sheet in the thickness direction and the average grain size of the ferrite phase serving as the main phase at the center position of the steel sheet in the thickness direction of the ferrite phase serving as the main phase at the center position of the steel sheet in the thickness direction is 2 μm or less, in which the difference ΔV between the fraction (percent by volume) of a second phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (percent by volume) of the second phase at the center position of the steel sheet in the thickness direction is 2% or less, and in which the minimum lath interval a bainite phase or a tempered martensite phase at the position 1 mm from the surface of the steel sheet in the thickness direction is 0.1 μm or more.
Patent Literature 3 reports a technique for providing a hot-rolled steel sheet having a tensile strength TS of 760 MPa or more in terms of strength and a fracture transition temperature vTrs of −100° C. or lower in terms of toughness, the hot-rolled steel sheet having a composition that contains, on a mass %, 0.03% to 0.06% C, 1.0% or less Si, 1% to 2% Mn, 0.1% or less Al, 0.05% to 0.08% Nb, V: 0.05% to 0.15% V, 0.10% to 0.30% Mo, and the balance being Fe and incidental impurities, and the hot-rolled steel sheet having a microstructure which is composed of a bainite single phase and in which carbonitrides of Nb and V are dispersed in the bainite phase in an amount of 0.06% or more in terms of the total amount of Nb and V.
Regarding techniques relating to heavy steel plates unlike hot-rolled steel sheets, Patent Literature 4 reports a technique for providing a high-strength steel sheet having low yield ratio and excellent uniform elongation characteristics, the steel sheet having a composition that contains, on a mass % basis, 0.06% to 0.12% C, 0.01% to 1.0% Si, 1.2% to 3.0% Mn, 0.015% or less P, 0.005% or less S, 0.08% or less Al, 0.005% to 0.07% Nb, 0.005% to 0.025% Ti, 0.010% or less N, 0.005% or less O, and the balance being Fe and incidental impurities, the steel sheet having a two-phase microstructure including bainite and an M-A constituent, and the M-A constituent having an area ratio of 3% to 20% and a circle equivalent diameter of 3.0 μm or less.
Patent Literature 5 reports a technique: a method for producing a heavy high-strength hot rolled steel sheet with excellent strength-ductility balance, the method including heating a steel and subjecting the steel to hot rolling including rough rolling and finishing rolling, the steel containing, on a mass % basis, 0.02% to 0.08% C, 0.01% to 0.50% Si, 0.5% to 1.8% Mn, 0.025% or less P, 0.005% or less S, 0.005% to 0.10% Al, 0.01% to 0.10% Nb, 0.001% to 0.05% Ti, and the balance being Fe and incidental impurities, C, Ti, and Nb being contained in such a manner that ([% Ti]+([% Nb]/2))/[% C]<4; performing accelerated cooling including primary accelerated cooling and secondary accelerated cooling, the primary accelerated cooling being performed in such a manner that a temperature at a position 1 mm from a surface of a sheet in the thickness direction is lowered to a primary cooling stop temperature of 650° C. or lower and 500° C. or higher at an average cooling rate of 10° C./sec. or more at a center position of the sheet in the thickness direction and in such a manner that a difference in cooling rate between the average cooling rate at the center position of the sheet in the thickness direction and an average cooling rate at the position 1 mm from the surface of the sheet in the thickness direction is less than 80° C./sec, and the secondary accelerated cooling being performed in such a manner that a temperature at the center position of the sheet is lowered to a secondary cooling stop temperature equal to or lower than BFS (° C.)=770-300C-70Mn-70Cr-170Mo-40Cu-40Ni-1.5CR (CR: cooling rate (° C./sec.)) at an average cooling rate of 10° C./sec. or more at the center position of the sheet in the thickness direction and in such a manner that a difference in cooling rate between the average cooling rate at the center position of the sheet in the thickness direction and the average cooling rate at the position 1 mm from the surface of the sheet in the thickness direction is 80° C./sec. or more; and after the second accelerated cooling, performing coiling at a coiling temperature equal to or lower than BFS0 (° C.)=770-300C-70Mn-70Cr-170Mo-40Cu-40Ni at the center position of the sheet in the thickness direction.